Microlithographic projection exposure methods and systems are currently used to fabricate semiconductor components and other finely patterned components. A microlithographic exposure process involves using a mask (reticle) that carries or forms a pattern of a structure to be imaged. The pattern is positioned in a projection exposure system between an illumination system and a projection objective in a region of the object surface of the projection objective. Primary radiation is provided by a primary radiation source and transformed by optical components of the illumination system to produce illumination radiation directed at the pattern of the mask in an illuminated field. The radiation modified by the mask and the pattern passes through the projection objective, which forms an image of the pattern in the image surface of the projection objective, where a substrate to be exposed is arranged. The substrate normally carries a radiation-sensitive layer (photoresist).
When a microlithographic projection exposure system is used in the manufacture of integrated circuits, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the integrated circuit. This pattern can be imaged onto an exposure area on a semiconductor wafer which serves as a substrate.
In many applications the projection objective is designed as a reduction projection objective forming a demagnified image of the pattern on the substrate at a magnification ratio |β|<1, for example at 4:1 (|β|=0.25) or 5:1 (|β|=0.2) reduction. Unit magnification projection objectives (|β|=1) may also be used. Magnifying projection objectives with magnification |β|>1 may be used, for example, in the manufacturing of liquid crystal display panels or other large micro structured components.
Projection exposure is performed at a given image-side numerical aperture NA appropriately selected for the specific type of pattern to be imaged. While a projection objective is designed with regard to aberration correction etc. to allow a specific maximum image-side numerical aperture (design NA), the effective numerical aperture actually used in an exposure process is normally defined by a mechanical aperture stop arranged at or close to a pupil surface of the projection objective, i.e. at a position which is in Fourier transform relationship to the image surface of the projection objective. A non-variable aperture stop with a fixed diameter of the aperture opening may be employed. Variable aperture stops allowing to vary the diameter of the aperture opening are employed in many cases, thereby allowing to set for specific applications the effective image-side numerical aperture to values smaller than the maximum possible image-side NA of the projection objective.
A variable aperture stop at a pupil surface of the projection objective involves a relatively well corrected pupil in order to ensure that changes in the effective numerical aperture actually used for an exposure by stopping up or stopping down the aperture stop have substantially the same effect for all field points of the field to be imaged (field-constant effect). Further, a mechanical aperture stop involves installation space in the region of the pupil surface. Therefore, no refractive or reflective optical surface of an optical element should be in the region of the pupil surface. Further, positions optically close to a pupil surface may be preferred positions of pupil filter elements and/or adjustable manipulation devices for deliberately (actively) changing the imaging properties of a projection objective. Therefore, it may be difficult to provide a variable or non-variable mechanical aperture stop at an appropriate pupil position of the projection objective.
A pattern of a mask may include different types of partial patterns. For example, a line pattern with densely packed parallel lines may be present in one portion of a pattern, and isolated features, such as contact holes, may be present in another portion of a mask. While imaging of line patterns with small pitch may involve a relatively high NA for imaging with sufficient resolution, isolated features may be imaged best with relatively lower NA values, for example in order to increase the depth of focus (DOF) of the projection objective. It may be difficult to find a suitable compromise NA to image both dense lines and isolated features with sufficient quality.
Further, due to the increasing demands on the efficiency of the lithographic manufacturing process there is a tendency to increase the power of the light sources. Also, progressively shorter wavelengths are used. Specific illumination settings are employed to optimize the imaging conditions for various pattern types. As a result, various time-dependent changes in the properties of optical materials and other components within the projection system are observed, which may sensibly affect the imaging quality of the exposure system. Non-uniform heating of lens groups and other transparent optical elements (“lens heating”) during operation due to an increased absorption in parts of the optical system is one effect dynamically influencing the imaging properties.